1. Field of the Disclosure
The present disclosure relates to an antenna device, and relates in particular to (but is not limited to) a wireless communication apparatus that adopts a MIMO (Multi Input Multi Output) antenna device.
2. Description of Related Art
A service called “LTE” (Long Term Evolution) is known as one of specifications for high-speed data communication for cellular phones. When seen from a technical viewpoint for an antenna, the LTE has the following characteristics.
That is, the LTE is intended for a communication system called “MIMO”, in which a plurality of antennas are used for transmission and reception to achieve high-speed data communication. A wireless communication apparatus such as a portable terminal that adopts the MIMO normally uses two antennas. It is desired that the two antennas should ideally have equal antenna characteristics.
An index called “correlation” between the antennas serves as a key point of the antenna characteristics for the MIMO. It is known that a large value (coefficient) of correlation (that is, a high degree of correlation) between the antennas reduces the communication speed.
The frequency band that is currently or planned to be used for the LTE service in various countries exists over a wide range, and it is desired that both a lower band and a higher band of an existing cellular system should be broadened.
For example, in a service in 700-MHz band in the United States, it is significantly difficult to reduce the correlation between the antennas. This is because a lower frequency causes a high-frequency current to flow through the entire substrate of a portable terminal, which brings the antennas into an operation mode similar to that of dipoles to make the directivity of the antennas less dependent on the design of the antennas. Hence, if it is attempted to improve the correlation by varying the directivity by changing the design of one of the antennas, it is considerably difficult to obtain desired results.
Japanese Unexamined Patent Application Publication No. 2008-17047 proposes a multi-antenna that can be applied to a mobile communication system that is less affected by mutual coupling. The multi-antenna includes a plurality of power feed elements respectively connected to a plurality of power feed points on a circuit substrate, and a single or a plurality of parasitic elements connected to the circuit substrate in the vicinity of an arbitrary power feed point.